This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The purpose of this project is to explore the role of Natural Killer (NK) cells in promoting in utero engraftment by adult hematopoietic stem cells. NK cells play a role in overcoming immunologic barriers to hematopoietic stem cell engraftment in adult animals. We have demonstrated that immunologic barriers are present in the fetus that can interfere with engraftment of adult stem cells early in gestation. We are examining the ability of adult NK cells to overcome these immune barriers and improve in utero engraftment by adult hematopoietic stem cells.